Mass spectrometry is an analytical technique for accurate determination of molecular weights, the identification of chemical structures, the determination of the composition of mixtures, and qualitative elemental analysis. In operation, a mass spectrometer generates ions of sample molecules under investigation, separates the ions according to their mass-to-charge ratio, and measures the relative abundance of each ion.
Time-of-flight (TOF) mass spectrometers separate ions according to their mass-to-charge ratio by measuring the time it takes generated ions to travel to a detector. TOF mass spectrometers are advantageous because they are relatively simple, inexpensive instruments with virtually unlimited mass-to-charge ratio range. TOF mass spectrometers have potentially higher sensitivity than scanning instruments because they can record all the ions generated from each ionization event. TOF mass spectrometers are particularly useful for measuring the mass-to-charge ratio of large organic molecules where conventional magnetic field mass spectrometers lack sensitivity. The prior art technology of TOF mass spectrometers is shown, for example, in U.S. Pat. No. 5,045,694 and 5,160,840 specifically incorporated by reference herein.
TOF mass spectrometers include an ionization source for generating ions of sample material under investigation. The ionization source contains one or more electrodes or electrostatic lenses for accelerating and properly directing the ion beam. In the simplest case the electrodes are grids. A detector is positioned a predetermined distance from the final grid for detecting ions as a function of time. Generally, a drift region exists between the final grid and the detector. The drift region allows the ions to travel, in free flight, a predetermined distance before they impact the detector.
The flight time of an ion accelerated by a given electric potential is proportional to its mass-to-charge ratio. Thus the time-of-flight of an ion is a function of its mass-to-charge ratio, and is approximately proportional to the square root of the mass-to-charge ratio. Assuming the presence of only singly charged ions, the lightest group of ions reaches the detector first and are followed by groups of successively heavier mass groups.
In practice, however, ions of equal mass and charge do not arrive at the detector at exactly the same time. This occurs primarily because of the initial temporal, spatial, and kinetic energy distributions of generated ions. These initial distributions lead to broadening of the mass spectral peaks. The broadened spectral peaks limits the resolving power of TOF spectrometers.
The initial temporal distribution results from the uncertainty in the time of ion formation. The time of ion formation may be made more certain by utilizing pulsed ionization techniques such as plasma desorption and laser desorption. These techniques generate ions during a very short period of time.
An initial spatial distribution results from ions not being generated in a well-defined plane perpendicular to the flight axis. Ions produced from gas phase samples have the largest initial spatial distributions. Desorption techniques, such as plasma desorption or laser desorption ions, result in the smallest initial spatial distributions because ions originate from well defined areas on the sample surface and the initial spatial uncertainty of ion formation is negligible. The initial energy distribution results from the uncertainty in the energy of the ions during formation. A variety of techniques have been employed to improve mass resolution by compensating for the initial kinetic energy distribution of the ions. Two widely used techniques use an ion reflector (also called ion mirror or reflectron) and pulsed ion extraction.
Pulsed ionization such as plasma desorption (PD) ionization and laser desorption (LD) ionization generate ions with minimal uncertainty in space and time, but relatively broad initial energy distributions. Conventional LD typically employs sufficiently short pulses (frequently less than 10 nanoseconds) to minimize temporal uncertainty. However, in some cases, ion generations may continue for some time after the laser pulse terminates causing loss of resolution due to temporal uncertainty. Also, in some cases, the laser pulse generating the ions is much longer than the desired width of mass spectral peaks (for example, several IR lasers). The longer pulse length can seriously limit mass resolution. The performance of LD may be substantially improved by the addition of a small organic matrix molecule to the sample material, that is highly absorbing, at the wavelength of the laser. The matrix facilitates desorption and ionization of the sample. Matrix-assisted laser desorption/ionization (MALDI) is particularly advantageous in biological applications since it facilitates desorption and ionization of large biomolecules in excess of 100,000 Da molecular mass while keeping them intact.
In MALDI, samples are usually deposited on a smooth metal surface and desorbed into the gas phase as the result of a pulsed laser beam impinging on the surface of the sample. Thus, ions are produced in a short time interval, corresponding approximately to the duration of the laser pulse, and in a very small spatial region corresponding to that portion of the solid matrix and sample which absorbs sufficient energy from the laser to be vaporized. This would very nearly be the ideal source of ions for time-of-flight (TOF) mass spectrometry if the initial ion velocities were also small. Unfortunately, this is not the case. Rapid ablation of the matrix by the laser produces a supersonic jet of matrix molecules containing matrix and sample ions. In the absence of an electrical field, all of the molecular and ionic species in the jet reach nearly uniform velocity distributions as the result of frequent collisions which occur within the jet.
The ion ejection process in MALDI has been studied by several research groups. R. C. Beavis, B. T. Chait, Chem. Phys. Lett., 181, 1991, 479. J. Zhou, W. Ens, K. G. Standing, A. Verentchikov, Rapid Commun. Mass Spectrom., 6, 1992, 671-678. In the absence of an electrical field, the initial velocity distributions for peptide and protein ions produced by MALDI are very nearly independent of mass of the analyte and laser intensity. The average velocity is about 550 m/sec with most of the velocity distribution between 200 and 1200 m/sec. The velocity distribution for matrix ions is essentially identical to that of the peptides and proteins near threshold irradiance, but shifts dramatically toward higher velocities at higher irradiance. The total ion intensity increases rapidly with increasing laser irradiance, ranging from about 10.sup.4 ions per shot near threshold to more than 10.sup.6 at higher irradiance. In the presence of an electrical field, the ions show an energy deficit due to collisions between ions and neutrals. This energy deficit increases with both laser intensity and electrical field strength and is higher for higher mass analyte ions than it is for matrix ions.
The observation that the initial velocity distribution of the ions produced by MALDI is nearly independent of mass implies that the width of the initial kinetic energy distribution is approximately proportional to the square root of the mass as well as the energy deficit arising from collisions with neutral particles in the accelerating field. Thus the mass resolution, at high mass, in conventional MALDI decreases with the increasing mass-to-charge ratio of the ions. Use of high acceleration potential (25-30 kV) increases the resolution at high mass in direct proportion to the increase in accelerating potential.
The adverse effect of the initial kinetic energy distribution can be partly eliminated by pulsed ion extraction. Pulsed or delayed ion extraction is a technique whereby a time delay is introduced between the formation of the ions and the application of the accelerating field. During the time lag, the ions move to new positions according to their initial velocities. By properly choosing the delay time and the electric fields in the acceleration region, the time of flight of the ions can be adjusted so as to render the flight time independent of the initial velocity to the first order.
Considerable improvements in mass resolution were achieved by utilizing pulsed ion extraction in a MALDI ion source. Researches reported improved resolution as well as fast fragmentation of small proteins in J. J. Lennon and R. S. Brown, Proceeding of the 42nd ASMS Conference on Mass Spectrometry and Allied Topics, May 29-Jun. 3, 1994, Chicago, Ill., p. 501. Also, researchers reported significant resolution enhancement when measuring smaller synthetic polymers on a compact MALDI instrument with pulsed ion extraction in Breuker et al., 13th International Mass Spectrometry Conference, Aug. 29-Sep. 3, 1994. Breuker et al., 13th International Mass Spectrometry Conference, Aug. 29-Sep. 3, 1994, Budapest, Hungary. In addition, researchers reported considerably improved mass resolution on small proteins with a pulsed ion extraction MALDI source in Reilly et al. Rapid Commun., Mass Spectrometry, 8, 1994, 865-868. S. M. Colby, T. B. King, J. P. Reilly, Rapid Commun. Mass Spectrom., 8, 1994, 865-868.
Ion reflectors (also called ion mirrors and reflectrons) are also used to compensate for the effects of the initial kinetic energy distribution. An ion reflector is positioned at the end of the free-flight region. An ion reflector consists of one or more homogeneous, retarding, electrostatic fields. As the ions penetrate the reflector, with respect to the electrostatic fields, they are decelerated until the velocity component in the direction of the field becomes zero. Then, the ions reverse direction and are accelerated back through the reflector. The ions exit the reflector with energies identical to their incoming energy but with velocities in the opposite direction. Ions with larger energies penetrate the reflector more deeply and consequently will remain in the ion reflector for a longer time. In a properly designed reflector, the potentials are selected to modify the flight paths of the ions such that ions of like mass and charge arrive at the detector at the same time regardless of their initial energy.
The performance of a mass spectrometer is only partially defined by the mass resolution. Other important attributes are mass accuracy, sensitivity, signal-to-noise ratio, and dynamic range. The relative importance of the various factors defining overall performance depends on the type of sample and the purpose of the analysis, but generally several parameters must be specified and simultaneously optimized to obtain satisfactory performance for a particular application.
Unfortunately, utilizing the prior art techniques, the performance of TOF mass spectrometers is inadequate for analysis of many important classes of compounds. These inadequacies are particularly apparent with MALDI. There are several mechanisms that may limit the performance of TOF mass spectrometry in addition to the loss of mass resolution associated with the initial kinetic energy distribution. An excess of generated matrix ions may cause saturation of the detector. Due to a long recovery time of many detectors, saturation seriously inhibits the true reproduction of the temporal profile of the incoming ion current which constitutes essentially the TOF spectrum.
Fragmentation processes have been observed to proceed at three different time scales in MALDI TOF, E. Nordhoff, et al., J. Mass Spectrom., 30 1995, 99-112. Extremely fast fragmentation can take place essentially during the time of the ionization event. This process is referred to as prompt fragmentation. The fragment ions will give a correlated ion signal in a continuous ion extraction MALDI TOF measurement, that is, fragment ions behave exactly as if they were present in the sample. Fragmentation can also take place at a somewhat lower rate during the acceleration stage (typically with less than one .mu.sec characteristic time). This kind of fragmentation is referred to as fast fragmentation. High energy collisions (more energetic than thermal collisions) between ions and neutrals can also contribute to fast fragmentation. These collisions are particularly frequent in the early stage of ion acceleration when the ablated material forms a dense plume. Fragment ions from the fast fragmentation processes, as opposed to prompt fragments, contribute to uncorrelated noise (chemical noise) since they will be accelerated to a wide range of kinetic energies unlike the original sample ions which are accelerated to one well-defined kinetic energy.
Fragmentation of sample ions may also occur in the free-flight region which occurs on a longer time scale comparable with the flight time of the ions. This may or may not be desirable depending on the particular type of data that is required from the time-of-flight mass spectrometer. Generally, fragmentation decreases the intensity of the signal due to the intact molecular ions. In mixture analysis, these fragment ions can produce significant chemical noise which interferes with detection of the signals of interest. Also, fragmentation within a reflector further reduces the intensity of the signal of interest and further increases the interfering background signal.
When fragmentation occurs in a drift region, except for the very small relative velocity of the separating fragments, both the ion and neutral fragment continue to move with nearly the same velocity as the intact ions and arrive at the end of the field-free region at essentially the same time, whether or not fragmentation has occurred. Thus in a simple TOF analyzer, without reflector, neither the resolution nor the sensitivity is seriously degraded by fragmentation after acceleration.
On the other hand, in the reflecting analyzer the situation is quite different. Fragment ions have essentially the same velocity as the intact ions, but having lost the mass of the neutral fragment, have proportionally lower energy. Thus the fragment ions penetrate a shorter distance into the reflecting field and arrive earlier at the detector than do the corresponding intact ions. By suitable adjustment of the mirror potential these fragment ions may be focused to produce a high quality post-source decay (PSD) spectrum which can be used to determine molecular structure.
It is therefore a principal object of this invention to improve the performance of time-of-flight mass spectrometers, particularly in regard to applications involving production of ions from surfaces, by improving resolution, increasing mass accuracy, increasing signal intensity, and reducing background noise. It is another object to reduce the matrix ion signal in MALDI time-of-flight mass spectrometers. Another objective is to provide TOF mass spectrometers suitable for fast sequencing of biopolymers such as nucleic acids, peptides, proteins, and polynucleotides by the analysis of chemically or enzymatically generated ladder mixtures. Still another objective is to utilize fast fragmentation processes for obtaining structural information on biomolecules such as oligonucleotide, carbohydrates, and glycoconjugates. Yet, another objective is to control the extent of fast fragmentation by selecting the most appropriate experimental conditions in a pulsed ion extraction TOF mass spectrometer.